In order to develop a preclinical model of gene therapy using CD34+ hematopoietic stem cells, we compared various methods of obtaining and engrafting CD34+ stem cells in rhesus monkeys. We investigated rhG-CSF and rhSCF for their ability to mobilize CD34+ cells into the peripheral circulation. We collected these by plasmapharesis. We also collected CD34+ cells from bone marrow by ficoll separation. The latter cells were transduced with a retroviral vector containing a reporter gene, and were reinfused into the donor monkeys. Monkeys to be reinfused receive 200 cGy total body irradiation in order to purge the marrow and improve the chances of engraftemnt. The first monkeys to have transduced cells reinoculated have shown only slight transgene expression.